Path diversity receiver apparatus and method of spectrum spread communication system

ABSTRACT

A path detector ( 100 ) provides statistics of rates at which preamble signals are detected within a predetermined time period. For a delay amount segment where the detection rate is low, the sampling rate of correlation value calculation is reduced, and the number of correlator sets used for the preamble signal detection is reduced. Correlator sets made available by that reduction are used to detect message signal paths, thereby raising the operating rate of the RAKE receivers of a decoder ( 200 ) to improve the throughput of the message signals. Further, the statistics of the rates of preamble signal detections are periodically updated, and when the preamble signal detection rate of the delay amount segment where the sampling rate was reduced becomes higher, the number of the correlator sets used for the preamble signal detection is increased so as to return the sampling rate to its original value.

TECHNICAL FIELD

The present invention relates to a path diversity reception device withrandom access using preamble signals in a spread spectrum communicationsystem, and particularly, to the realization of a high-throughputcommunication method in a mobile communication system.

BACKGROUND ART

In a mobile communication system using CDMA (Code Division MultipleAccess) which has become increasingly common in recent years, randomaccess is used for one of uplink channels from a mobile station to abase station. For example, in IMT-2000 (DS-CDMA), a PRACH (PhysicalRandom Access Channel) corresponds thereto. Channel models andcommunication protocols of the PRACHs, which are described in documentssuch as 3GPP TS25.211, etc., use preamble signals as signalsfor,conveying message signal transmission permission request from amobile unit to a base station, and message signals for transmittinginformation from an actual mobile unit.

Here, it is throughput that is important in transmitting/receivingPRACHs which are first transmitted when a mobile unit is powered on, andwhen calling as well as when packet data is transmitted. This is becausepoor PRACH throughput causes the user to feel inconvenienced by ‘notreadily getting through’. The simplest way to enhance this throughput isto increase the number of simultaneous decoding, i.e., the number ofallowable simultaneous decoding of PRACH message signals in a basestation, as an alternative to reducing signal errors to reduce thenumber of times of retransmission.

To increase the number of simultaneous decoding, however, from the pointof view of device scale and cost, it is not very well advisable tosimply increase the number of a path detection correlator set as in FIG.1 and RAKE receivers as in FIG. 2. The correlator set is used not onlyfor message signal path detection, but also for preamble signaldetection. In detecting message signal paths, a delay profile isselectively obtained in the vicinity of a delay amount in which arereceived preamble signals, whereas, in detecting preamble signals,because of no such preliminary information, the use of a correlator setto obtain a delay profile is required for all of up to a maximum delayamount matched to a cell radius. Therefore, as the cell radius is largeras in FIG. 3, more correlator sets are required for preamble signaldetection, while the number of correlator sets that can be used formessage signal path detection is limited. As a result, the number ofRAKE receivers that can be operative in the decoder is also decreased.

Accordingly, it is an object of the present invention to provide a pathdiversity reception device and method for a spread spectrumcommunication system with improved throughput by efficiently using asignal-processing portion of the path diversity reception device whenimplementing random access using preamble signals in the spread spectrumcommunication system such as a CDMA system, etc., used in mobilecommunications, etc.

DISCLOSURE OF INVENTION

According to the present invention, in order to solve the aboveproblems, a path diversity reception device for a spread spectrumcommunication system takes statistics of frequency at which preamblesignals within constant time are detected by a path detector, drops asample rate of correlation value calculation in a delay amount intervalin which detection frequency is low, and decreases the number ofcorrelator sets used in detecting preamble signals, while the remainingcorrelator sets are used in detecting message signal paths, therebyincreasing an operation rate of RAKE receivers of a decoder, andenhancing the throughput of message signals. Statistics of detectionfrequency of preamble signals are regularly updated, and when preamblesignal detection frequency in a delay amount interval in which thesample rate has been dropped is increased, the number of the correlatorsets used in detecting preamble signals is controlled to be increased torestore the sample rate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing one example of a conventional path detectioncorrelator set;

FIG. 2 is a diagram showing one example of a conventional RAKE receiver;

FIG. 3 is a diagram showing the relationships between cell radii and thenumber of simultaneous decoding;

FIG. 4 is a functional block diagram of an essential portion forexplaining the present invention;

FIG. 5 is a block diagram of a baseband signal-processing portion in abase station;

FIG. 6 is a diagram showing an example of the correspondences betweencorrelator sets and delay amount intervals;

FIG. 7 is a flowchart for explaining PRACH calling processing sequence;and

FIG. 8 is a flowchart for explaining processing sequence concerningcorrelator set management.

BEST MODE FOR CARRYING OUT OF THE INVENTION

The preferred embodiments according to the invention will be explainedbelow referring to the drawings.

FIG. 4 is a functional block diagram of an essential portion forexplaining the present invention. FIG. 4 shows an example in which thepresent invention is applied to a CDMA mobile communication system, andin which are depicted functional blocks of a portion of a basebandsignal-processing portion when a base station receives PRACHs (PhysicalRandom Access Channels) as described in 3GPP TS25.211 V3.8.0, chapters5.2.2.1 and 7.3 which defines the standards of Physical Channels ofIMT-2000 (DS-CDMA).

In FIG. 4, a received-signal-processing portion of the base station ofthe CDMA mobile communication system comprises a path detector 100 forpreamble signal detection and message signal path detection, and adecoder 200 for decoding message signals. The path detector 100comprises correlator sets 101-112, and detects preamble signals andpaths for receiving message signals. The decoder 200 comprises RAKEreceivers 201-212, and performs path diversity reception of messagesignals, using path information notified from the path detector 100.

Since preamble signal detection requires processing for all of up to amaximum value of a delay amount determined by a cell radius, as the cellradius is larger, the number of correlator sets used for preamble signaldetection is increased. In FIG. 4, the correlator sets 101-108 are usedfor preamble signal detection, while the correlator sets 109-112 areused for message signal path detection.

On the other hand, in the decoder 200, since maximum 4 message signalpath information is simultaneously notified from the path detector 100,the corresponding 4 RAKE receivers 209-212 are used in decoding themessage signals. For this reason, the RAKE receivers 201-208 remainunused. Since the number of these unused RAKE receivers is increased asthe cell radius is larger and the number of the correlator sets used forpreamble signal detection is increased in the path detector 100, it isdesirable to be able to use these RAKE receivers in building a system,if possible.

The present invention takes statistics of frequency at which preamblesignals within constant time are detected in the path detector 100,drops a sample rate of correlation value calculation in a delay amountinterval in which detection frequency is low, and decreases the numberof the correlator sets used in detecting preamble signals, while theremaining correlator sets are used in detecting message signal paths,thereby increasing an operation rate of the RAKE receivers of thedecoder 200, and enhancing the throughput of the message signals.Statistics of detection frequency of preamble signals are regularlyupdated, and when preamble signal detection frequency in a delay amountinterval in which the sample rate has been dropped is increased, thenumber of the correlator sets used in detecting preamble signals iscontrolled to be increased to restore the sample rate.

FIG. 5 is a block diagram of a baseband signal-processing portion in abase station including the functional blocks depicted in FIG. 4.

FIG. 6 shows an example of the correspondences between correlator setsand delay amount intervals when preamble signals are detected in thepath detector 100 of FIG. 4.

FIG. 7 is a flowchart for explaining PRACH calling processing sequencein the present embodiment. Reception baseband signals 901 demodulatedbeforehand by a demodulator are input to a path detector 100 shown inFIG. 5. In step S1, the path detector 100 first obtains a delay profileof a delay amount corresponding to a cell radius, and in step S2,detects preamble signals by determining that the preamble signals aredetected when a correlation value peak exceeds a certain threshold (apreamble threshold).

When obtaining a delay profile, in the present embodiment, as shown inFIG. 6, the entire delay amount is divided into short delay amountintervals for every 8 chips. For each interval, 2 correlator sets areused to calculate correlation values at a ½-chip sampling rate. In thiscase, the correlator sets used for every ½ chip are alternately changed.In other words, the correlator sets 101 and 102 of FIG. 4 arealternately used in delay amount interval A of 0 to less than 8 chips,and the correlator sets 103 and 104 are alternately used in delay amountinterval B of 8 to less than 16 chips. (Although omitted in FIG. 6, thecorrelator sets 105 and 106 are alternately used for every ½ chip indelay amount interval C of 16 to less than 24 chips, and the correlatorsets 107 and 108 in delay amount interval D of 24 to less than 32chips.)

When preamble signals are detected in the path detector 100 in step S3,its detection information and information on a delay amount in which thepreamble signals are detected, i.e., preamble signal detectioninformation 902 is notified to a control device 300 in step S4. Thecontrol device 300 always manages the use of the correlator sets of thepath detector 100. When the detection of preamble signals is notified,the control device 300 first determines whether there are remainingcorrelator sets assigned to message signal path detection in step S5.When there are remaining correlator sets, the control device 300 sendscode generation information 903 to a code generator 400 to transmit to amobile unit ACK signals for permitting the transmission of messagesignals in step S6. On the contrary, when there are no remainingcorrelator sets, the control device 300 sends code generationinformation 903 to the code generator 400 to transmit to the mobile unitNACK signals for prohibiting the transmission of message signals in stepS12. Actually, according to the code generation information 903, thecode generator 400 generates transmission baseband signals 904indicating ACK or NACK signals, which are sent to a modulator.

When sending ACK signals to the mobile unit in step S6, the controldevice 300 subsequently sends correlator set setting information 905 tothe path detector 100, sets spread codes and timing for one of thecorrelator sets reserved for message signal path detection, and startsmessage signal path detection processing in step S7. The control device300 also sends RAKE receiver setting information 906 to a decoder 200 toset spread codes, data formats, etc in step S8.

The path detector 100 directed to perform message signal path detectionprocessing by the correlator set setting information 905 performs pathdetection of message signals transmitted from the mobile unit, withdesignated correlator sets and timing in step S9. In this case, a delayprofile is selectively obtained in the vicinity of a delay amount inwhich are received preamble signals, and a high correlation valueposition is searched.

Path delay amount information found in this manner is notified to thedecoder 200 as path information 907. This information is set in one ofthe RAKE receivers within the decoder 200 to decode the message signalsin step S10. And, message data 908 obtained is sent via the controldevice 300 to a host device 500 in step S11.

FIG. 8 is a flowchart for explaining processing sequence concerningcorrelator set management in the present embodiment. The control device300 shown in FIG. 5 manages the correlator sets within the path detector100 after or in parallel with PRACH calling processing. When preamblesignals are detected, the control device 300 first determines in whichdelay amount interval the preamble signals are detected from delayamount information contained in preamble signal detection information902 sent from the path detector 100 in step S21. And, the number oftimes of reception is tabulated while detecting the preamble signals foreach delay amount interval in constant time in step S22. Afterprescribed time has passed in step S23, loop processing for each delayamount interval is performed in steps S24-S28. This loop processing foreach interval is iterated from i=0 to i=(delay amount interval number−1)sequentially adding 1 to i. For each interval, by dividing the number oftimes of reception of the preamble signals by the total number of timesof sampling of the preamble signals within that time, preamble signaldetection frequency for each delay amount interval is obtained in stepS25. For a delay amount interval in which its result is lower than apredetermined threshold in step S26, the number of the correlator setsused in preamble signal detection is controlled to be decreased to lowerthe sample rate of correlation value calculation. This controlinformation is sent to the path detector 100 as correlator set settinginformation 905.

This sequence of controls allows surplus correlator sets to be used formessage signal path detection. The preamble signal detection frequencyon which is based this control is updated for every constant time. Evenif the sample rate is temporarily lowered at preamble signal detectionfrequency below a threshold, if this detection frequency goes above thethreshold, the sample rate can be restored into the original state againby adding correlator sets. This control is also performed by the controldevice 300 sending control information to the path detector 100 ascorrelator set setting information 905.

For example, in FIG. 4, for the correlator sets 101-112 within the pathdetector 100 and the RAKE receivers 201-212 within the decoder 200, itis assumed that, as in the initial state A, the correlator sets 101-108are used for preamble signal detection, the correlator sets 109-112 areused for message signal path detection, and the RAKE receivers 209-212are used in decoding message signals. Here, in the decoder 200, themessage signal decoding is performed only by the 4 RAKE receivers209-212. This is because the 4 correlator sets 109-112 send only theirrespective corresponding 4 message signal path information.

In this state, preamble signals are detected using the correlator sets101-108. After constant time has passed, preamble signal detectionfrequency for each delay amount interval is obtained. A result such asgraph G1 is assumed to be obtained. As seen from graph G1, since thepreamble signal detection frequency is below the threshold th in theintervals A and D, the respective numbers of the correlator sets incharge of the intervals A and D are decreased by 1. In other words, asshown in the state B of FIG. 4, the preamble signal detections in theintervals A and D are respectively performed by only the correlator sets101 and 107, while the surplus correlator sets 102 and 108 are used formessage signal path detection. This allows the number of the correlatorsets used for message signal path detection in the path detector 100 tobe increased. As a result of that increase, the number of the RAKEreceivers that can be operative for message signal decoding in thedecoder 200 can also be increased.

Since the numbers of the correlator sets used in the intervals A and Dare decreased, on the other hand, the sample rate for delay profilepreparation is changed from ½ chip to 1 chip. However, because it hasbeen found from the preamble signal detection frequency that the numbersof preamble signals detected in these intervals are small, the number ofmobile units affected by it is small. Even if the mobile unit transmitspreamble signals in these intervals, the preamble signals are notentirely undetectable in the base station, and the extent of the problemis such that since an accurate correlation value peak cannot be obtaineddue to the decrease of the sample rate, detection probability isdecreased. Because the PRACHs standardized in 3GPP TS25.211 haveprotocols for control of preamble signal retransmission and transmissionpower control during its retransmission, preamble signals can bedetected in the base station by several times of retransmission even inthe worst case.

Next, in the layout of the correlator sets of the state B, preamblesignals are detected, and preamble signal detection frequency for eachdelay amount interval is calculated. Preamble signal detection frequencydistribution such as graph G2 is assumed to be obtained. Because thepreamble signal detection frequency is above the threshold th in theintervals A and D, the numbers of the correlator sets in these intervalsare controlled to be restored to change the sample rate for a delayprofile from 1 chip to ½ chip. In other words, the setting modificationis such that the correlator sets 102 and 108 which have been used formessage signal path detection are used for preamble signal detection inthe original intervals A and D.

To this end, since path information from the path detector 100 no longerreaches the RAKE receivers 202 and 208 which have decoded the messagesignals in the decoder 200, these RAKE receivers are restored into theoriginal unused states.

The present invention is not limited to the above embodiment, but may beapplied to the following systems:

-   -   (1) A mobile communication system other than IMT-2000 (DS-CDMA);    -   (2) A system where a mobile station side serves as a reception        side in random access;    -   (3) A subscriber radio access system and an autonomous        distributed system where no base station control is performed;    -   (4) A system where path diversity reception is performed without        CDMA;    -   (5) A system where surplus RAKE receivers according to the        effect of the present invention are used not for PRACH message        signals, but for other channel (voice and data dedicated        channel) reception; and    -   (6) A system having a path diversity reception function but no        random access as in the embodiment, and which can control the        number of correlators and sample rate for path detection        according to path detection frequency.

The numbers of the correlator sets and RAKE receivers, delay amountinterval width, sample rate values, etc., shown in the above embodiment,may be modified according to system and environment.

Furthermore, as an extension of the present invention, instead of thepreamble signal detection frequency, delay amount magnitude may beconsidered in controlling the correlator sets so as to facilitate thereception of preamble signals in a large delay amount position. This isbecause of avoiding the following: Since a mobile unit with a largedelay amount, i.e., a mobile unit distant from a base station musttransmit preamble signals with large power compared with a nearby mobileunit, retransmitting preamble signals of the distant mobile unit manytimes causes large interference with signals of other mobile units. Inother words, in the large delay amount, even if the preamble signaldetection frequency is low, without simply decreasing the number of thecorrelator sets, the sample rate is controlled so as not to be lowered.

Industrial Applicability

According to the present invention, frequency at which preamble signalsare detected is calculated, and the number of the correlator sets usedin detecting the preamble signals is adaptively changed according to itsdetection frequency, thereby increasing the operation rate of the RAKEreceivers of the decoder, and allowing the throughput of the messagesignals to be enhanced.

1. A path diversity reception device for a spread spectrum communicationsystem, comprising: a path detector comprising a plurality of correlatorsets for detecting preamble signals or paths of message signals; adecoder comprising a plurality of RAKE receivers being providedcorresponding to said each correlator set for using path informationfrom said path detector for decoding said message signals; and a controlportion for controlling these, wherein: said control portion comprises:control means for calculating frequency at which preamble signals aredetected within constant time in said path detector, for decreasing thenumber of correlator sets used in detecting said preamble signals in adelay amount interval in which said detection frequency is low, and forlowering a sample rate of correlation value calculation, and settingmeans for using said decreased correlator sets in detecting paths ofmessage signals or other channel signals and using its path informationwith said corresponding RAKE receivers for decoding said message signalsor other channel signals.
 2. The path diversity reception deviceaccording to claim 1, wherein: detection frequency of said preamblesignals is regularly calculated, and when said preamble signal detectionfrequency is increased in a delay amount interval in which said samplerate has been lowered, the number of correlator sets used in detectingsaid preamble signals is controlled to be increased to restore saidsample rate.
 3. The path diversity reception device according to claim1, wherein: in a delay amount interval in which said preamble signaldetection frequency is low, without decreasing the number of saidcorrelator sets in an interval in which a delay amount is larger than aconstant value, said sample rate is controlled so as not to be lowered.4. The path diversity reception device according to claim 2, wherein: ina delay amount interval in which said preamble signal detectionfrequency is low, without decreasing the number of said correlator setsin an interval in which a delay amount is larger than a constant value,said sample rate is controlled so as not to be lowered.
 5. A pathdiversity reception method for a spread spectrum communication systemwith a path diversity reception device that comprises a path detectorcomprising a plurality of correlator sets for detecting preamble signalsor paths of message signals, and a decoder comprising a plurality ofRAKE receivers being provided corresponding to said each correlator setfor using path information from said path detector for decoding saidmessage signals, said method comprising the set of: calculatingfrequency at which preamble signals are detected within constant time insaid path detector, decreasing the number of correlator sets used indetecting said preamble signals in a delay amount interval in which saiddetection frequency is low, and lowering a sample rate of correlationvalue calculation; and performing a setting for using said decreasedcorrelator sets in detecting paths of message signals or other channelsignals and using its path information with said corresponding RAKEreceivers for decoding said message signals or other channel signals. 6.The path diversity reception method according to claim 5, wherein:detection frequency of said preamble signals is regularly calculated,and when said preamble signal detection frequency is increased in adelay amount interval in which said sample rate has been lowered, thenumber of correlator sets used in detecting said preamble signals iscontrolled to be increased to restore said sample rate.
 7. The pathdiversity reception method according to claim 5, wherein: in a delayamount interval in which said preamble signal detection frequency islow, without decreasing the number of said correlator sets in aninterval in which a delay amount is larger than a constant value, saidsample rate is controlled so as not to be lowered.
 8. The path diversityreception method according to claim 6, wherein: in a delay amountinterval in which said preamble signal detection frequency is low,without decreasing the number of said correlator sets in an interval inwhich a delay amount is larger than a constant value, said sample rateis controlled so as not to be lowered.